(a) Field of the Invention
Exemplary embodiments relate to an aerostatic air bearing, an assembling method thereof, and an aerostatic lead screw actuator using the aerostatic air bearing.
(b) Description of the Related Art
Miniaturization technologies have placed new challenges on the world of manufacturing. Micro-scale machine tools (mMTs) have recently emerged as a viable and economic option to address these challenges.
However, evolving applications for mMTs have begun to include materials that are difficult to machine. For example, hard turning and micro-machining of titanium, stainless steels, and bulk metallic glass are just some challenges faced by mMTs.
These applications require increased static/dynamic stiffness and high damping in the actuators used in mMTs. A prior solution for linear positioning, ball screws, does not generally achieve the positioning accuracy required for mMTs.
Hydrostatic lead screw actuators and aerostatic lead screw actuators have been developed for use in mMTs. Hydrostatic screws are messy and require significant support equipment.
Aerostatic lead screw actuators (ALSAs) have the potential to provide the necessary stiffness as well as extremely high positioning accuracy. ALSAs use an air film in place of balls to transmit the load from the nut (aerostatic lead screw actuators (ALSAs)) to the screw. ALSAs also eliminate backlash and stick-slip friction and therefore appear to be particularly suitable for high performance mMTs. The major challenge posed in the manufacture of an ALSA, however, is maintaining the precise air gap between the screw and the nut over the entire helical threaded surface.
In the art, both orifice-restricted and porous-restricted air bearings have been incorporated in aerostatic lead screw designs. For example, Tachikawa et al., “Ultra Precision Positioning Using Air Bearing Lead Screw,” Transactions of the Japan Society of Mechanical Engineers, Vol. 66, No. 645, pp. 1559-1566, 2000, disclose a porous-restricted aerostatic lead screw actuator that achieves 10 nm positioning accuracy. However, this lead screw actuator design provides relatively low stiffness (e.g., 30 N/micron), and requires thread engagement with eight thread revolutions, which significantly increases manufacturing costs.
A design and manufacturing method for a groove-fed orifice-restricted ALSA is disclosed in U.S. Pat. No. 4,836,042 by Slocum et al. Due to the special nature of the thread design in the '042 patent, however, any inaccuracy in the thread profile doubles the error seen in the air gap.
U.S. Pat. No. 5,090,265 by Slocum et al. discloses the use of a hydrostatic lead screw actuator that does not require precise air gap control. The manufacturing process is based on machining both the thread and the nut on the same machine to maximize the repeatability of cutting operations. In the '265 patent, which describes a design and manufacturing method for an orifice-restricted fluid-based lead screw actuator, high pressure air, oil, and other fluids provide a working fluid. The lead screw actuator focuses on orifice-restriction as the primary method to control air flow. These orifices feed grooves that distribute air to the entire thread flank of the lead screw to create an air gap between the nut and the screw.
However, such a focus on orifice-restriction to control airflow is highly susceptible to instability and vibration. Further, the lead screw actuator in the '265 patent is limited to a square thread form, which is not an industry standard. Also, using the manufacturing method presented in the '265 patent, any inaccuracy in the thread profile is doubled where the air gap is concerned. For example, if the thread flank has a slight angle creating an error of 0.5 microns at the outer diameter, this will create an error of 1 micron in the air gap. This property effectively forces the tolerance requirements on a screw made in this way to be halved, thereby increasing manufacturing costs. Still further, using fluids other than air for the '265 patent device requires considerable fluid support equipment, which is less desirable for machine tool applications such as MMT applications.